Archive for August 19th, 2017

Why not another variable sweep fighter?

There seems to be a lot of F-14 nostalgia around. While it may have had a great deal of impact on how the US Navy conducted fleet defense, we have to consider the effectiveness of the concept of variable sweep aircraft. It is human nature to always want to look up to the past. The other reason may very well be that people find the F-14 to look visually attractive and want similar proposals.

The reason why we will not see future variable sweep fighters however is because there are very serious drawbacks compared to fixed wing aircraft.

Short Background

Variable sweep wings, known as “Swing wing” evolved as a solution for early jet engines. Experiments were being made as early as WW2 with wings that could change their sweep on the ground, such as the Messerschmitt P.1101.

Back then jet engines produced less thrust because they ran at lower inlet temperatures and were overall more primitive. Wings with a sharp sweep were desired for high top speed, but that left the aircraft vulnerable in dogfights, which as Vietnam revealed still happened, and also led to high take-off and landing speeds. High take off and landing speeds are less safe, which would result in increased number of crashes. They also led to long runways, limiting off road mobility and making it easier to disable for enemy forces, as there would be a far larger airport to protect.

In Europe, there were two key projects, the Panavia Tornado, which entered service as a mult-role interceptor/bomber, and the Dassault Mirage G, which never entered production. The US would build the F-111, which was a very heavy variable sweep multi-role aircraft. The famous F-14 was derived from the F-111. The USSR made several variable sweep designs, most notably the Mig-23 and the Su-24.

Bomber designs were also made by the US and USSR. The B1 Lancer from the US, along with the Tu-22 and Tu-160 from the USSR. All 3 bombers remain in service.

What do swing wing aircraft bring?

Their main advantage is that they can use that variable sweep wing to find the optimal wing swing angle (within their sweep limits) for a given airspeed. This can allow for fuel savings on the climb and landing during a fighter sortie.

On aircraft carriers, they have the advantage of having very low sweep on take-off and very high sweep when bursting with full afterburner. Variable sweep wings can also be folded for compact storage without compromising wing’s structural integrity (as is the case with folding wings like on F-18E).

On an aircraft carrier, deck-space is always going to be a bottleneck. While a carrier may look very large to an untrained eye, deck space is always at a very big premium.

So why not on fighter aircraft?

To achieve variable sweep aircraft, that requires a large gearbox in the fuselage of the aircraft. This gearbox adds a great deal of mass and makes the fuselage larger, causing drag. This means that fuel fraction on such aircraft is lowered a great deal.

In a dogfight, this heavy gearbox would mean that compared to a fixed wing, it would result in an unfavorable thrust to drag, even if the pilot could switch to what they felt was the optimal sweep right before combat. Switching the wing sweep during a dogfight would be risky, as it could cause a loss of energy.

This would mean:

Higher wing loading due to mass of gearbox

Faster fuel consumption due to gearbox

Lower transient performance (very important in a dogfight)

This gearbox would also lead to lower G limits as well. On the F-14D, the symmetric limit at 50,000 lbs was 6.5G. The F-16 and F-15 were both capable of 9G. Navalized versions of the F-18 were capable of 7.5G, while certain land based variants of the F-18 could also perform 9G. For a comparison, Dassault Rafale can do 11G, with an ultimate limit of 16.5G.

The gearbox lowered the aircraft’s fuel fraction. An empty F-14D has a mass of 43,735 lb ( or about 19,838 kg) and can take on 16.200 lb of fuel. This results in a fuel fraction of 0,27, which is below 0,30 fuel fraction required for sufficient combat persistence.

Jet engines have become far more powerful than their 1960s and 1970s counterparts, allowing for much higher thrust to weight ratios. As such, they can achieve lower take distances, even more so on an aircraft carrier with a catapult. This fact somewhat negates swing-wing’s main advantage of high low-speed efficiency.

Modern computer control surfaces too have played a role in rendering variable wing sweep obsolete as they can adjust wing shape and size very rapidly, without the weight penalty.

The decision to incorporate the Super Hornet and decommission the F-14 is mainly due to high amount of maintenance required to keep the Tomcats operational. On average, an F-14 requires nearly 50 maintenance hours for every flight hour, while the Super Hornet requires five to 10 maintenance hours for every flight hour.

I’ve been told that a newer F-14 would likely require 40 to 1 and on average, the F-18 requires 8 to 1, which is in line with the USN’s claims of 5-10 to 1. So in that regard, the F-18 would be able to generate much higher sortie rates. Keep in mind that the 50 to 1 is with after the General Electric F110 engines were put on the F-14. Early F-14s suffered from an unreliable TF-30 engine that was prone to flame-outs.

Compounding the problem, the high flight to maintenance ratios mean that there’s a good chance you will not have enough F-14s available when you need them the most (ex: if an enemy launches a surprise attack on your carrier battle group, you may need to scramble the aircraft very quickly).

There were other points of failure. Sometimes when one side of the gearbox worked properly and the other did not, it could lead to an “asymmetric wing sweep”.

While the aircraft could fly in such a situation and land with some difficulty, this leaves a point of failure. This could also be a weakness in combat, as the hydraulics could be damaged.

Much like this F-14, under Australian service, the F-111 did encounter a similar incident, and the B1 did once as well. I suspect that under Warsaw Pact service, Soviet variable sweep designs may have too.

Conclusions

The cons simply outweigh the pros when it comes to variable wing sweep. There are very significant penalties in terms of mass, cost, and complexity for variable sweep wings. While they may bring some advantages in the take-off and can have the “optimal” sweep for each scenario, the drawbacks outweigh these to the point where we are not seeing variable wing sweep aircraft on modern aircraft.

They are simply a dead end as far as aircraft design goes. While they may have seemed like a good idea on paper, when implemented in combat aircraft, they carried significant drawbacks that outweighed any advantages they brought.